Pressure sensor

ABSTRACT

A pressure gauge is arranged to function on the outside of a measuring element. The measuring element has a central cavity and is constituted by at least to parts, which are tightly joined for creation of the cavity. The measuring element has sensor organs for determining the mechanical state of stress of the measuring element during pressure influence. The two parts of the measuring element are manufactured with planar techniques, preferably by silicon or quarts with the cavity running in the longitudinal direction. The central cavity has a considerably greater height than width. The sensor organs have form of piezo-resistive elements arranged near an outer surface of the measuring element.

The present invention concerns pressure sensors manufactured with planartechniques. The invention especially concerns a pressure gaugepreferably arranged to function on the outside of a measuring element.The measuring element has a central cavity running in the longitudinaldirection of the measuring element which is composed of at least twoparts which are tightly joined for creation of the cavity.Piezo-resistive elements are used to register the mechanical state ofstress of the measuring element during pressure influence.

BACKGROUND

A pressure gauge consists in principle of a spring element (measuringelement) is and a measuring or sensor organ. Common commerciallyavailable pressure gauges based on silicon technology may use membranesas the spring element, provided with piezo-resistive resistors as thesensor organs. Such membrane sensors have a typical size of 3 mm×4 mm.Spring elements like membranes are disadvantageous for high pressuresbecause such elements are susceptible for effects related to restrainingto a substrate with transition to materials with different modules ofelasticity. The tension detected on such membranes will be a combinationof pressure and tensile stresses. If the tensile stress becomesadequately high, breakage may occur. At large deformations, the stressin the membrane will not be linearly dependent of the pressure,providing a non-linear signal.

The closest prior art in this connection is described in NorwegianPatent NO 304 328 describing a pressure gauge particularly for very highpressures. The measuring principle that is used is described in the book“Instrumenteringsteknikk” by Ole A. Solheim, Tapir forlag Trondheim1966, and uses piezo-resistive elements arranged on the outside of themeasuring element for detecting tensile stress appearing in themeasuring element. The two parts of the measuring element aremanufactured with planar techniques, preferably of silicon or quartz,and have a considerably greater length than transverse dimension, withthe cavity running in the longitudinal direction. The pressure gauge is,however, especially suitable for measuring very high pressures.

The present invention represents a further development of the sensordescribed in the Norwegian Patent NO 304 328 to lower pressure ranges.

SUMMARY OF THE INVENTION

In a first aspect, the invention concerns a pressure gauge comprising ameasuring element having a longitudinal direction and a central cavitythat extends in this longitudinal direction. The measuring element isconstituted by at least two parts, manufactured in planar technique andpreferably of silicon or quarts, and they are tightly joined forcreation of the cavity. The two parts are joined in a joining plane. Themeasuring element comprises sensor devices in the form ofpiezo-resistive elements for sensing the mechanical state of stress ofthe measuring element under pressure influence. The piezo-resistiveelements are arranged on a surface of the two parts parallel with thejoining plane. The pressure gauge in accordance with the invention ischaracterized in that a cross section of the central cavity has aconsiderably greater height than width, the height of the cross sectionbeing defined as the direction perpendicular to the joining plane.Further, the width of the cross section is perpendicular to the height,and of the central cavity is at least of the magnitude five times thewidth.

In a preferred embodiment, the height of the central cavity is at leastof the magnitude five times the width. The central cavity may comprisetwo single canals running in the longitudinal direction of the measuringelement and which are arranged side by side. The cavity is constitutedby an upper part and a lower part, where the upper and the lower partsare complementary. The cavity may also be constituted of two partsproviding a cross section with effectively three layers. The threelayers constitute an upper part, a lower part and a central part, wherethe upper and lower parts are complementary, and the central part isinserted to increase the height of the cavity in relation to the width.The cavity may also in another embodiment be constituted of partscreating a cross section with effectively four layers.

In a further embodiment, the upper and the lower parts are identical inat least as concerns the cross sectional form and the cavity is createdby reactive ion-etching (RIE) in those parts of the surface facing theother part. The outer cross section form of the measuring element may,for example, be hexagonal, octagonal or decagonal and symmetrical abouttwo longitudinal planes, but alternatively it can also be shaped as ahexagonal and be symmetrical about two longitudinal planes. Two of theparts constituting the measuring element may also be identical in atleast as concerns the cross-sectional shape. The parts of the measuringelement may be joined by anodic bonding, or by so-called direct bonding.The pressure gauge can be designed for measuring full scale pressure inthe range 0.5-100 bar.

The pressure gauge according to the invention is stated in the appendedpatent claims.

The sensor provides large and robust signals which result in that it canbe connected to cheaper and simpler electronics. It will also have aconsiderably less area and thereby be cheaper to produce.

BRIEF DESCRIPTION OF DRAWINGS

The invention shall now in the following be further explained in theform of different example embodiments shown in the drawings, where:

FIG. 1 represents prior art and shows in perspective an embodiment of apressure gauge (measuring element) mounted in a holder;

FIG. 2A shows an enlarged cross section of the main part of themeasuring element in FIG. 1;

FIG. 2B shows in an enlarged cross section a support part of themeasuring element in FIG. 1, close to the holder;

FIG. 3 shows in a corresponding cross section as in FIGS. 2A and 2B, avariant of the cross section form of the measuring element, that is thecross section of the cavity;

FIG. 4 shows an enlarged cross section of the main part of the measuringelement according an embodiment of the invention designed for measuringlow pressures;

FIG. 5 shows an enlarged cross section of the main part of the measuringelement according to a further embodiment of the invention designed formeasuring high pressures; and

FIGS. 6-8 show variants of a cross section shape according to theinvention.

DETAILED DESCRIPTION

FIG. 1 shows the closest prior art and shows an example embodiment ofthe sensor in NO 304 328. An extended measuring element 1 is constitutedby two parts 1A and 1B which are identical and which together create aninternal cavity 3 between two end walls 3A and 3B. The cavity 3 does notdistend completely out to the ends of the main part of the measuringelement 1, which is here shown with an octagonal cross section.

A surface 5 of the measuring element is provided with sensor organs 11,12, 13 and 14, which through a number of conductors 15 are connectedwith external electronics or measuring circuits. The sensor organs 11,12, 13 and 14 have form of in themselves known piezo-resistive elementswhich preferably are arranged in a bridge connection. The sensor issensitive to changes in the mechanical state of tension of the measuringelement or deformation when it is exposed to varying pressure differencebetween the surroundings and the internal cavity 3.

At its internal end, the octagonal main part 1 of the measuring elementverges into a support part 2 having a quadratic external cross sectionprofile, aiming at interacting with a mounting piece 18 in which thewhole measuring element is mounted. The mounting piece 18 hasaccordingly a throughgoing hole 19, which is preferably circular, andwhich is adapted to a bushing part 7 on the measuring element. The part7 has advantageously the same external cross section profile as the mainpart 1. The support part 2 serves to give a safe anchoring of themeasuring element against the mounting piece 18, which has particularinterest when the measuring element is exposed to very high externalpressures. Such pressures will seek to press the measuring element 1 inaxial direction into the hole 19, but the outranging corners 2A, 2B, 2Cand 2D (see also FIG. 2A) will effectively block against displacement ofthe measuring element during such pressure influence. Through the hole19 the bushing part 7 may be further fixed by using appropriate gluefilling out the space between the octagonal cross section and thecircular hole 19.

FIGS. 2A, 2B and 3 show in more detail different embodiments of thecross section of the known measuring element in FIG. 1. The two parts 1Aand 1B are joined in the plane marked with 8. The four corners 2A, 2B,2C and 2D in FIG. 1A serve to anchor against the holder 18. Thequadratic external cross section profile on FIG. 2B corresponds to thecross section in the support part 2, but the internal cavity 3 does notreally exist in the support part 2 when absolute pressure measurement isconcerned, i.e., with a closed cavity 3 between end walls 3A and 3B asin FIG. 1. In that respect, FIG. 2B may be regarded illustrative of analternative main form of the effective length of a measuring element 1,where the cavity 3 may have the same cross section form as in FIG. 2Aand 1. It is thus favorable to design the cavity 3 in the knownmeasuring element with a rhombic cross section having two cornerslocated in the joining plane 8 between the two parts 1A and 1B.

FIG. 3 shows a variant where the outer cross section contour of themeasuring element 31 is the same as in FIG. 2A, that is octagonal,whereas the inner cavity 3 has a hexagonal cross section profile. Thisis, like the cavities 3, having a rhombic shape, well suited formanufacture in planar techniques, particularly by etching. In additionto a quadratic and octagonal outer cross section contour, the knownmeasuring element may be produced with a polygonal external crosssection profile in other variants, like a hexagonal cross section.

The known pressure gauge described above is, however, particularlyadapted to high pressures.

The present invention represents a further development of the pressuregauge described in the Norwegian Patent NO 304 328, and which isdescribed above. The pressure gauge according to the invention may beused in lower pressure ranges, that is in the area 0.5-100 bar. Theinvention consists mainly in a change of the geometry of the cavity'scross section, to a cavity which in cross section has essentially agreater height than width. Embodiments of the invention will now bedescribed.

FIG. 4 shows a first embodiment of the cavity in the measuring elementaccording to the invention. The cavity has a rectangular cross sectionform, but with essentially a greater height than width. The measuringelement is constructed by two parts 100A and 100B which are identicaland which together form the inner cavity 103. The external form of themeasuring element is in the embodiment shown in FIG. 4, polygonal. Thesurface 105 extends beyond the elongated side walls defining the heightof the measuring element. This construction provides space for thesensor organs on the outside of the measuring element as in the knownembodiment in FIG. 1. The minimum outer width of the measuring elementsmay be about 100 μm to be able to get enough space to be able to locateconductors 15. Here, also used are piezo-electrical elements in bridgeconnection as sensor organs as in the known embodiment in FIG. 1. Atypical size of the cavity will, in accordance with the presentinvention, be a height of 4-6 mm and a width of 200 μm, but the widthmay be reduced, for example, 100 μm. The height should be at least befive times the width of the cavity to be able to achieve the desiredeffect. The side walls in this modified cavity will, when the measuringelement is exposed to pressure influence, be forced inwards. This bringsabout that the outer surfaces of the two ends will cause considerablyinfluence on the sensor organs 15 arranged on the outside of themeasuring element on these end surfaces. The measuring element accordingto the invention is, however, still essentially longer than its width,and the length may be in the magnitude 100 times the width. Asmentioned, the width is determined to a great extent by the sensororgans which shall be arranged on the outside of the measuring element.If piezo-resistive elements are used as sensor organs, a typicalmagnitude of the signals in the pressure range 0.5-100 bar would beabout 40 mV/Vbar. This is an essential greater signal than what can beachieved with the known embodiment shown in FIGS. 1-3, which has amaximum of 1 mV/Vbar. The mounting of the measuring element may, forexample, be performed as shown in FIG. 1 for the known sensor designedfor very high pressures.

In FIG. 5, it is shown another embodiment of the cavity especially formeasuring higher pressures. Here, the relationship between the heightand width is reduced in relation to the embodiment shown in FIG. 4.

FIGS. 6 and 7 show alternative embodiments of the present invention. InFIG. 6, the height is increased by inserting a third intermediate part100C between the two main parts 100A and 100B. Such an intermediate partmay also be inserted in the known embodiment shown in FIGS. 1, 2A, 2Band 3 to achieve a considerably greater height than width. Thisembodiment is shown in FIG. 8. Here, an hexagonal inner cavity 203 isachieved, while the outer geometrical form becomes octagonal.

FIG. 7 shows another alternative embodiment of the present inventionwhere an intermediate part is included to increase the height of thecavity 103 in relation to the width of the measuring element. Here, theside walls of the intermediate part are thinner than the side walls ofthe upper and lower parts, but the inner shape of the cavity isrectangular. The outer geometrical form of the measuring element isbihexagonal. The height may be further increased by furtherintermediates inserted in between the upper 100B and lover 100A parts.The cross section of the measuring element will then be constituted byat least four layers.

In an alternative embodiment, the cavity 103 may be equipped with aninternal partition for creation of two cavities. The two cavities formtwo single canals which run side by side in the longitudinal directionof the measuring element. The height of the cavities must beconsiderably greater than the sum of the width of the two cavities.

The well shaped, deep cavity in the present invention can be produced bythe use of reactive ion-etching (RIE). This is a complexed techniquewhere well-formed depressions are first etched in the element, and thencoated with a protective film. This protective film is removed byphysical etching on selected locations and the cavity may thus be madedeeper. RIE is a known technique which makes it possible to etch arecess in a planar element where the depths are considerably greaterthan the width.

Based on planar technique, the at least two parts constituting themeasuring element may be joined as mentioned by, for example, use ofanodic bonding or so-called direct bonding (fusion bonding). The partscan also be joined by use of suitable glue.

The width of the measuring element is decided as mentioned by conductorsand wiring points which must have space on the outside of the narrowupper end surfaces of the element. This provides a lower limit for hownarrow the measuring element may be, and this limit is with today'stechnology about 100 μm.

In connection with mounting, packing and encapsulation of the measuringelement, it is clear that this element may be provided with a protectingfilm on the surface, an example given at Si₃N₄ possibly polyamideplastic, in such a way that the measuring element may be exposeddirectly for the actual pressure medium.

Several other modifications and variants are also possible within theframe of the invention, perhaps especially connected to the wish ofachieving rational and economical production processes. An example giventhe parts of the measuring element in themselves may consist of acomplex structure containing layers of different material types.Further, a possible modification may consist in that the sensor organscan be arranged on more than one surface of the measuring element, forexample, on two oppositely directed main surfaces.

1. A pressure gauge comprising: a measuring element comprising at leasttwo parts joined at a joining plane such that a central cavity runningin a longitudinal direction of said measuring element is formed; and aplurality of sensor devices operable to sense a mechanical state ofstress of said measuring element under pressure influence, saidplurality of sensor devices being arranged on a surface of saidmeasuring element parallel with the joining plane, wherein a crosssection of the central cavity has a height at least five times greaterthan a width, the height of the cross section being defined as adirection perpendicular to the joining plane, and the width of the crosssection being perpendicular to the height.
 2. A pressure gauge accordingto claim 1, wherein the central cavity comprises two single canalsrunning in the longitudinal direction of said measuring element andwhich are arranged side by side.
 3. A pressure gauge according to claim1, wherein said at least two parts is four parts, thereby creating across section with four layers.
 4. A pressure gauge according to claim1, wherein an external cross section shape of said measuring element ishexagonal, octagonal or decagonal and symmetrical about two longitudinalplanes.
 5. A pressure gauge according to claim 1, wherein the crosssection of the central cavity is rectangular and symmetrical about twolongitudinal planes.
 6. A pressure gauge according to claim 1, whereinthe cross section of the central cavity is hexagonal and symmetricalabout two longitudinal planes.
 7. A pressure gauge according to claim 1,wherein two parts of the at least two parts of the measuring elementhave a same cross section shape.
 8. A pressure gauge according to claim1, wherein said at least two parts of said measuring element are joinedby anodic bonding or by direct bonding.
 9. A pressure gauge according toclaim 1, wherein said measuring element and said plurality of sensordevices are adapted for measurement of full-scale pressures in a rangeof 0.5-100 bar.
 10. A pressure gauge according to claim 1, wherein saidplurality of sensor devices are piezo-resistive elements.
 11. A pressuregauge according to claim 1, wherein said at least two parts eachcomprise silicone or quartz.
 12. A pressure gauge according to claim 1,wherein said at least two parts are each manufactured with a planartechnique.
 13. A pressure gauge according to claim 1, wherein saidplurality of sensor devices are piezo-electric elements.
 14. A pressuregauge according to claim 1, wherein said at least two parts are an upperpart and a lower part, said upper part and said lower part beingcomplementary.
 15. A pressure gauge according to claim 14, wherein saidupper and lower parts have a same cross section profile and the centralcavity is created by reactive ion-etching surfaces of said upper andlower parts facing each other.
 16. A pressure gauge according to claim1, wherein said at least two parts is three parts, thereby creating across section with three layers.
 17. A pressure gauge according to claim16, wherein said three parts an upper part, a lower part and anintermediate part, said upper part and said lower part beingcomplementary, and said intermediate part being inserted to increase theheight of the central cavity in relation to the width.
 18. A pressuregauge according to claim 17, wherein said upper and lower parts have asame cross section profile and the central cavity is created by reactiveion-etching surfaces of said upper and lower parts facing each other.